1. Field of the Invention
The present invention relates to a voltage monitoring device for monitoring output voltages of an assembled battery in which a plurality of unit cells are connected in series so as to generate a desired voltage.
2. Background Art
For example, a hybrid vehicle has a high voltage battery serving as a driving power supply for the motor. In the high voltage battery, for example, a plurality of unit cells for secondary battery (storage battery) such as nickel-hydrogen battery cells and lithium battery cells are connected in series so as to provide a high voltage.
Further, in order that the secondary battery should be prevented from being overcharged or overdischarged, the charging state of each unit cell need to be checked. Thus, in the background art, a plurality (e.g., 55) of unit cells are divided into, for example, five blocks (i.e., each 11 unit cells corresponds to one block). Then, the voltages outputted from the unit cells of each block are measured in real time by an IC for voltage detection provided in each block. Then, it is determined whether the measured voltage of each unit cell falls within a predetermined range (see, for example, JP-A-2002-84669).
FIG. 4 is an explanation diagram showing an example of a voltage detection circuit used in a background art voltage monitoring device. In the voltage detection circuit 101, a switching circuit 102 selects a unit cell so as to provide its voltage to an amplifier 103. Then, a comparator 104 compares a voltage outputted from the amplifier 103 with a reference voltage generated by voltage division by resistors R101 and R102, and on the basis of this comparison result, determines whether the output voltage of the unit cell is not less than a predetermined upper limit threshold voltage or alternatively not more than a predetermined lower limit threshold voltage.
Nevertheless, in the background art voltage monitoring device, since the reference voltage is generated on the basis of the ratio of the resistors R101 and R102, when the upper limit threshold voltage or the lower limit threshold voltage used in detection of the voltage of each unit cell are to be changed, the ratio of the resistors R101 and R102 need be adjusted. That is, there must be much work and cost in the adjustment of the upper limit threshold voltage or the lower limit threshold voltage.
The threshold value for determining the upper limit voltage or the lower limit voltage of each unit cell is changed in some cases depending on the installation environment such as the ambient temperature or on the type of the secondary battery to be connected. Then, in such cases, the circuit configuration need to be changed. This causes much cost in changing the components and much work of replacing the components for the worker.